Many Internet Service Providers (ISPs) offer multiple levels of service, charging a different fee for each service level. For example, a subscriber of an ISP may subscribe to e-mail only, connection access (i.e., Telnet and FTP), or full access which would allow access to the World Wide Web (WWW).
Typically, the ISPs are configured so that a subscriber cannot use a service which is not included in the subscriber's subscription. The Network Access Servers (NAS), which provide point-of-presence dial-in access for the ISPs, may provide filtering to prohibit a subscriber from accessing services not included in the subscriber's subscription. When a NAS detects a subscriber attempting to exceed their allowed service level, the NAS typically discards the subscriber's packet (which contains a request for the service), and returns a simple "cannot connect" type message to the subscriber.
As a result, the subscriber is uninformed as to why they are unable to connect or receive their requested service. As such, it would be desirable to return an appropriate application level message to the subscriber providing a more detailed explanation why the requested service is unavailable to the subscriber.